The present invention relates generally to semiconductor contact metallurgies, and more particularly to a Schottky barrier diode and ohmic contact metallurgy especially designed for use with shallow-junction semiconductor devices.
There is a continuing need for increasing the frequency of operation and the switching speed of bipolar semiconductor devices. Shallow junction bipolar semiconductor devices have been found especially suited for such high frequency operation. An important circuit element required for use with the shallow-junction semiconductor devices in order to facilitate such high frequency operation is the Schottky barrier diode. However, a problem with current Schottky diode technology is that such diodes do not maintain a constant predetermined barrier voltage height, but rather have a barrier height which varies as a function of the number of heat treatments utilized in fabricating the device. Specifically, each additional heat treatment applied to a given semiconductor chip causes the various metals in the Schottky diode metallurgy to diffuse into the underlying semiconductor, thereby changing the metal/semiconductor atomic ratios and thus the barrier height voltage for the diodes.
A further limiting factor is that there is a strong preference for depositing the metallurgy for the ohmic contacts at the same time as the metallurgy for the Schottky barrier diode in order to save process steps. Thus, the Schottky diode metallurgy must be compatible with ohmic contact device environments.
This compatibility requirement is a major stumbling block in shallow-junction semiconductor device processing. This is because many of the ohmic contacts to shallow-junction devices are disposed so that they are separated from underlying pn junctions in the device by only a thin semiconductor layer. The above-described metal-semiconductor interdiffusion in such an ohmic contact can thus electrically short through to the underlying pn junction. By way of example, a shallow-junction high frequency double-diffused NPN transistor might have an emitter region which is diffused into the base region to a shallow depth of only approximately 1,500 to 2,000 angstroms. Due to the small geometry of this shallow double-diffused transistor, the edge of the emitter diffusion opening in the oxide mask layer thereof is so close to the emitter-base junction at the surface of the wafer that horizontal migration of the interdiffusion between the metal and the semiconductor caused by the alloying process of the metallurgy can electrically short out the base-emitter junction. Likewise, because the base-emitter junction is shallow relative to the emitter contact diffusion opening in the oxide mask, vertical migration of the interdiffusion between the metal and the semiconductor can also electrically short out the emitter-base junction. Moreover, if this diffusion does not cause a direct short, then it facilitates a low breakdown voltage characteristic and high leakage currents in the ohmic contacts.
Aluminum, with a barrier height of 700 mV, is a prime candidate as a Schottky metal. However, aluminum in direct contact with an underlying semiconductor material is especially prone to the above-described interdiffusion due to its rapid diffusion characteristic and the high solubility of semiconductors therein at elevated temperatures.
Various solutions have been advanced in the prior art to solve this aluminum-semiconductor interdiffusion problem. A typical solution is disclosed in U.S. Pat. No. 4,316,209 wherein the aluminum metallurgy is alloyed with at least one noble metal from the group of Pd and Pt and at least one region of this metallurgy is alloyed with silicon. The resulting metallurgy is a ternary alloy of either Al.sub.3 Pd.sub.4 Si or Al.sub.3 Pt.sub.4 Si. This metallurgy consumes only approximately 1/7 of the amount of silicon as prior art Schottky metallurgies. However, this metallurgy has the previously described problem that each additional heat treatment causes the ratio of the Al to the Pd to the Si to change, thereby changing the barrier voltage height in the Schottky device.
The invention as claimed is intended to remedy the above described constant barrier height problem for Schottky diodes subjected to repeated heat treatments, while preventing the penetration of the Schottky metallurgy all the way through to the shallow junction of the semiconductor device. The advantages offered by the present invention are that a Schottky metallurgy structure and a method for fabricating that structure are provided which prevent the electrical shorting of shallow-junction semiconductor devices by the interdiffusion of the Schottky metallurgy with the semiconductor material of the device. The Schottky diode metallurgy of the present invention permits the fabrication of shallow-junction bipolar semiconductor devices with metallurgies which are especially prone to interdiffusion with the underlying semiconductor material of the bipolar device. In particular, the Schottky diode metallurgy permits the fabrication of shallow junction semiconductor devices with aluminum Schottky diodes without the electrical shorting of the junction which normally occurs with such a combination.